1. Field of the Invention
This invention relates to a coordinate input apparatus. More particularly, the invention relates to a coordinate input apparatus in which vibration generated by an vibration pen is sensed by vibration sensors provided on an vibration transmitting panel so that coordinates designated by the vibration pen may be sensed. The invention further relates to a vibration sensing device and a method of evaluating the device.
2. Description of the Related Art
Ordinarily, in an apparatus of this type, the times required for the vibration generated by the vibration pen to reach vibration sensors provided at prescribed positions of the vibration transmitting panel are measured, and the distances from the vibration pen to the vibration sensors and, hence, the coordinates designated by the vibration pen, are calculated based upon the measured values.
In order to indicate that vibration has reached a vibration sensor, it is necessary to produce a timing signal, which indicates the arrival of vibration at the vibration sensor, from the signal generated by the vibration sensor upon detection of vibration. The main processor that supervises the apparatus receives the vibration-arrival timing signal that is based upon the detection of vibration by each vibration sensor. In response to reception of the timing signal, the main processor completes the measurement to arrival of the vibration and executes the calculation of coordinates.
How to generate this timing signal is a problem. The practice used to generate the timing signal in the prior art is to measure either a transmission lag time that corresponds to the group velocity of vibration or a transmission lag time that corresponds to group velocity and phase velocity. In any case, in order to perform an accurate measurement of transmission lag time that corresponds to the group velocity, the envelope of a signal waveform sensed in the vibration sensor is produced, the peak position or inflection-point position of the envelope is detected and a timing signal is produced based upon the position detected. The envelope is differentiated in order to produce the timing signal corresponding to the peak position. In order to produce the timing signal corresponding to the inflection point, the envelope is subjected to differentiation processing twice. The period of time from the timing at which vibration is generated by the vibration pen to the initial zero-cross point of the first-order differentiated waveform or second-order differentiated waveform is measured as the transmission lag time of vibration.
Detection of the inflection point (second-order differentiation) is preferred over detection of the peak position (first-order differentiation) of the envelope because the position of the inflection point is located ahead of the peak position on the time axis. If unnecessary reflected waves from, say, the end face of the vibration transmitting panel should happen to become superimposed directly on the sensed waves with a slight offset, the earlier detected point is less susceptible to the effects of the reflected waves. If the earlier detected point is adopted, therefore, the vibration transmitting panel and, hence, the apparatus itself, can be made small in size.
It was considered that the circuitry for performing the above-mentioned differentiation demonstrates the same differentiation processing performance (an advance of .pi./2 in the phase of the output with respect to an input sinusoidal wave) with regard to input signals having a frequency less than a certain determined frequency fh. For this reason it was preferred that the frequency fh be higher than the frequency of the leading-edge portion of the envelope. Furthermore, it was so arranged that the frequency at which the amplification factor of the differentiating circuit becomes 0 dB (unity gain) is made to agree with the frequency of the envelope waveform so that there will be no change in the input/output amplitude ratio of the differentiating circuit.
However, a problem which arises is that the differentiation characteristic, i.e., the transmission phase characteristic, of the differentiating circuit is such that, in actuality, there is a phase advance of .pi./2 with respect to a signal whose frequency f satisfies the relation f&lt;&lt;fh, with the amount of advance in phase becoming gradually smaller as f increases and approaches fh. Consequently, the zero-cross point of the first-order differentiated waveform does not necessarily correspond to the peak of the input signal (envelope), nor does the zero-cross point of the second-order differentiated waveform necessarily correspond to the inflection point of the input signal.
In other words, with regard to the point of detection of vibration corresponding to the group velocity of vibration, no evaluation is made to determine the particular position on the envelope waveform to which this point corresponds before the envelope waveform is differentiated. The reason for this is that when differentiation processing is performed by the differentiating circuit, the peak position of the envelope is judged to be the detection point (the first zero-cross point of the differentiated signal) when this position is detected in the first order differentiation whereas the inflection point is judged to be the detection point when this point is detected in the second order differentiation. In seeking the vibration detection point in accordance with a detection-point evaluation method, described below, the inventors have clarified that even though detection of the inflection point of the envelope is intended by second order differentiation, in actuality all that is detected is the proximity of the peak position. As a consequence, vibration sensed by the vibration sensors is strongly influenced by reflected waves from the edge of the vibration transmitting panel, as a result of which errors in coordinate detection cannot be reduced.
Another problem is that when the advance in phase caused by differentiation is less than .pi./2, the zero-cross point of the differentiated waveform is shifted back and forth along the time axis owing to a fluctuation in the level of the input signal. More specifically, when the detection level at a vibration sensor fluctuates owing to distance from the vibration pen or a change in input writing pressure applied to the pen, the measured value of transmission lag time changes. This leads to erroneous detection of coordinates.